The present invention relates to a linear actuator which is provided with a first level of position control and a second level of position and overload control and, more particularly, to an improved drive mechanism having an anti-lockup feature for transferring rotary force from an electric motor to axially move a connected extension rod of the linear actuator to move a load.
Linear actuators are typically utilized in situations where a thrust force is used for applying linear motion. Examples of the utilization of such thrust force is in the operation of lever arms, cranks, slides and valve levers in industrial equipment. Such actuators are utilized for alternately moving objects between predetermined positional limits. The actuators can be utilized for moving the movable member between positions within such predetermined limits by the utilization of appropriate feedback means.
FIG. 1 is a cross-sectional view of a preferred embodiment of the linear actuator described in my above-identified copending U.S. patent application, wherein the linear actuator 10 includes a body housing 12 formed with an upper compartment 14, an intermediate motor casing opening 16, and a body tube opening 18 at the lower end thereof. The upper compartment 14 is closed by a cover plate 20 which is sealed to the body housing 12 by a cover gasket 22. A gear compartment face plate 24 is provided for sealing the drive gear mechanism 26 within the body housing 12. A face plate gasket 28 is provided for this purpose. Suitable socket head screws are provided to secure the cover plate 20 and face plate 24 to the body housing 12.
The internal operation of the linear actuator 10 is shown in FIG. 1, wherein electric drive motor 44 which is retained within motor casing 46 provides rotary power to a drive pinion gear 48 which in turn transmits power through an intermediate gear 50 to the main drive gear 52 which is journalled to the end of the drive screw 54 by a Woodruff key 56. Rotation of drive screw 54 moves the drive nut 58 axially within the body tube 60. The body tube assembly 62 is formed by the external body tube 60 which is fitted into opening 18 and housing 12, the internal drive screw 54, the drive nut 58 and an extension rod 64 affixed to the outer side of drive nut 50 so as to extend beyond the end cap 70 of the body tube assembly 62. The drive nut 58 is secured against rotation by reaction surfaces such as described in my copending application which are formed internally within body tube 60. Drive nut 58 may be a square-sided nut as disclosed in my copending application and drive screw 54 is shown coaxially centered within the extension rod 64.
Extension rod 64 is thus axially extendible beyond the end cap 70 of the body tube assembly 62. This extension rod 64 is secured at the outer end of the body tube assembly 62 by an end cap 70 which is formed from a non-ferrous metal which then acts as a bushing and a seal.
The driven end of drive screw 54 is supported by a pair of angular contact bearings 72 and 74 which are supported within a bearing opening within body housing 12. A main gear spacer 78 is provided between the main drive gear 52 and the two bearings 72 and 74. The main drive gear is secured to the end of the drive screw 54 by a flex nut 80. The intermediate gear 50 is retained on a dowel pin 82 which is journalled between bearings 84 and 86. This intermediate gear 50 has an outer teeth set 88 for contacting the drive pinion gear 48 and an inner set 90 for contact with the main drive gear 52.
The motor drive shaft 92 connected to a rotor core 93 is supported by a front bearing 94 within body housing 12 and at the outer end by a bearing 96 which is retained in the motor casing 46. The motor stator 98 is secured within motor casing 46 and is provided with a thermal sensing element 100 which together with the switch 102 forms an overload controller means 103 shown schematically. The thermal sensing element 100 can indirectly control the switch 102 as shown. Also, the thermal sensing element 100 and switch 102 can preferably be combined into a single bi-metallic switch such as disclosed in U.S. Pat. No. 3,219,856 to Dunwiddie.
Motor casing 44 is sealed within opening 16 and housing 12 by an O-ring.
A capacitor sub-assembly 104 is provided within compartment 14 in order to provide for change of phase between the windings in motor 44 to effect the instant reversal of direction of rotation. Motor 44 is preferably a single phase motor and is connected to the capacitor sub-assembly by a connection terminal 106 as shown. The drive pinion gear, intermediate gear and the main gear then comprise the drive means of the linear actuator.
During operation of the axial movement of extension rod 64 between the terminal stroke limits, the electric drive motor 44 is utilized to provide rotational power through the drive gear mechanism 26 so that rotational power is delivered to drive screw 54. A limit switch assembly (not shown) described in detail in my copending application can be set so that power to the electric drive motor 44 is interrupted just prior to the drive nut 58 reaching either of the two terminal positions which limit its stroke. In the event that the limit switches fail, the drive nut 58 will come into contact with either the back stop 108 or the front stop 110. Mating back stop reaction shoulder portion 112 is provided on drive nut 58 to provide a complementary abutment to the reaction shoulder 114 on the back stop 108. Back stop 108 is secured to the inner end of drive screw 54 by a set screw 116 which rests in a mounting slot 118. A similar reaction shoulder 120 is provided for front stop 110 for co-action with a mating reaction shoulder portion 122 secured to the front face of drive nut 58. Both the back stop 108 and the front stop 110 are secured to and rotate with the drive screw 54. A retaining flex nut 124 is provided for retaining front stop 110.
The abutment shoulders 114 and 120 and the shoulder portions 112 and 122 on the drive nut 58 thus function to restrain the movement of drive nut 58 relative to drive screw 54 so that the actuator stroke mechanism which is provided by the body tube assembly 62 is not jammed at the ends of the extension rod stroke when the power to the drive motor 44 has not been interrupted by the limit switch assembly. In such an event, as illustrated in FIG. 1, the drive nut 58 will come into abutment contact with the back stop 108 with a safety gap 126 remaining between the abutment shoulder extension 112 and the back stop 108. Continued operation of drive motor 44 will cause the stator coils 98 to heat up beyond the predetermined temperature which is sensed by the thermal element 100. The overload controller means 103 then operates to disengage the electric power supply to motor 44.
In the event that the extension rod 64 is prevented from movement during the axial movement of drive nut 58, this same overheating of the motor stator winding will occur which will then result in the electric power being interrupted from the drive motor 44. Thus, the overload control means 103 functions both at the terminal limits of the axial movement of extension rod 64 as well as within those limits in the event of an overload thrust condition.
The extension rod 64 is fitted with a load connector 125 which has internal threads 127 for connecting with the load (not shown). A clevis bracket 128 (not shown in FIG. 1) on the opposite end of the linear actuator provides a pivotal connection to a reaction support surface. The clevis bracket is secured to the gear compartment face plate 24 by socket head cap screws 130 and 132 which are balanced by a corresponding cap screw pair (not shown).
Linear actuators such as the type described above usually include the electric motor which is connected to the drive screw through a drive means which utilizes either a gear train or a drive belt. In the case of a drive mechanism in the form of a gear train, and with reference to my actuator described above, the drive pinion gear 48 is mounted upon an output shaft of drive motor 44. The intermediate gear 50 which includes a larger diameter gear 51 press-fitted onto a hub portion 50a thereof includes the outer teeth set 88 in direct mesh with drive pinion gear 48. In turn, the teeth of intermediate gear 50 are in direct mesh with main drive gear 52 for imparting axial movement to drive nut 58 and thereby extension rod 64.
One problem present in my aforesaid linear actuator and in other linear actuators having a drive mechanism in the form of a gear train is that upon initially actuating the drive motor 44 to longitudinally advance the drive nut 58 through the gear train 48,50,52 there is a tendency for the drive mechanism to "lockup" since the motor drive shaft 92 has not attained a speed approaching operating speed and therefore may not have developed sufficient torque to overcome the inertia of the driven system, i.e., the gear train 48,50,52, drive screw 54, drive nut 58, extension rod 64 and an output load, if any, connected to and driven by the linear actuator. Frequently, "lockup" occurs when the linear actuator connected to an output load (e.g., a hopper containing fine material) is idle for any length of time. Lockup may also occur in the event that operation of the linear actuator stops at a time when the drive nut is at an extreme end of stroke position under which the drive nut tends to wedge with respect to the drive screw 54. Obviously, one disadvantage of the lockup is the inability of the linear actuator to perform its intended function resulting in down time and loss of production until the problem has been corrected.